The present invention relates to electrical connecting elements, and more particularly to differential-mode connecting elements for use in transmission lines.
Communication networks transfer information, such as data, voice, text or video information, among communication devices connected to the networks. Most recent developments in communication technologies have been motivated by a desire to increase the available bandwidth of such communication networks to ever increasing levels. In addition, even the local communications associated with a computing device, such as communications on the read and write channels of a computer storage hard disk are also increasing, with bandwidth requirements currently approaching 1 GHz.
The need for such increasing bandwidth levels requires the electronic systems that participate in such communications to likewise operate at higher frequencies. The increased data rates required of such electronic systems requires a corresponding increase in the stringent requirements on the interconnection of active devices, passive devices, and package elements, such as integrated circuit elements within a semiconductor device. The traditional interconnection method of wire bonding does not meet the required electrical performance for broadband devices, primarily due to the bond wire inductance. Specifically, the wires used in such wire bonding techniques cause an inductance that attenuates the transferred signal levels at the required data rates. Therefore, wire bond interconnection poses a significant technical hurdle to overcome if broadband communication systems will be produced.
A number of interconnection techniques have been proposed or suggested that attempt to overcome such series inductance. For example, PCT Application Number WO 99/40627, assigned to GIGA A/S of Skovlunde, Denmark, discloses a flexible electrical connecting element 100, shown in FIG. 1, that is formed as a coplanar wave-guide on a dielectric substrate 105. A signal-carrying conductor path 110 and conductor paths 120, 130 on each side of the signal-carrying conductor path 110 constitute a ground plane. The electric fields in the conventional connecting elements 100 shown in FIG. 1 are created in the gaps 150, 160 between the conductor paths 110, 120, 130.
While such flexible electrical connecting elements 100 have significantly reduced the series inductance problem associated with conventional wire bond interconnection techniques, and perform effectively for many communication applications, they suffer from a number of limitations, which if overcome, could further reduce the overall dimensions and improve the impedance matching characteristics of such flexible electrical connecting elements. Specifically, high-speed electronics typically rely on integrated circuits (ICs) utilizing differential mode signal transmission for improved performance, relative to common mode signal transmission. In a differential signal mode, two lines, referred to as + and xe2x88x92, are required. Each of the data lines will have opposite current and opposite voltage at the same point on the line. To accomplish differential mode signal transmission using the electrical connecting elements 100 shown in FIG. 1, however, two such electrical connecting elements 100 are required. Alternatively, as shown in FIG. 2, a differential mode connecting element 200 can be accomplished using two signal-carrying lines 210, 220 and two ground lines 230, 240. Either of these constructions requires additional area to accomplish the interconnection. A need therefore exists for a flexible electrical connecting element that allows differential mode signal transmission to be employed without significantly increasing the required surface area.
In addition, another important characteristic of connecting elements used in transmission lines is the characteristic impedance of the interconnect 100. For most high data rate applications, transmission lines with characteristic impedances of 25 to 75 ohms are increasingly common. It is often a challenge, however, to obtain flexible interconnects that satisfy the impedance matching demands of high date rate communication devices. With the flexible interconnects shown in FIG. 1, desired impedance properties can be obtained by varying the width and gaps between of the conductor lines 110, 120, 130 and the dielectric constant of the dielectric substrate 105. Very fine gaps between the conductor lines 110, 120, 130 are typically required to achieve the required characteristic line impedance.
The accurate patterning of these gaps requires fine line lithography of the conductor 110. A small error in the conductor pattern will change the characteristic impedance of the flexible interconnect 100 and reduce the transmission bandwidth. Thus, the desired impedance strongly influences the geometry and material properties of the conventional flexible interconnects 100. A need therefore exists for a new flexible interconnect that offers additional degrees of freedom for varying the characteristic impedance.
Generally, an electrical connecting element is disclosed that is comprised of a dielectric substrate having a first conductor path (positive/+) on a first side and a second conductor path (negative/xe2x88x92) on a second side, substantially aligned with the first conductor path. The electrical connecting element employs differential-mode signaling such that the first conductor path carries a signal of opposite polarity to the second conductor path. The present invention recognizes that a virtual ground exists between the differential + and xe2x88x92 lines for a differential mode transmission line. The presence of the virtual ground permits a xe2x80x9cgroundlessxe2x80x9d differential transmission line. In addition, the substantial alignment of the first and second conductor paths improves the space constraints, relative to conventional electrical connecting elements.
According to another aspect of the invention, the characteristic impedance of the disclosed differential transmission line depends on the thickness and dielectric constant of the dielectric substrate and the width of the trace, which is significantly larger than the gap of the conventional flexible interconnect discussed above. Therefore, the required resolution of the conductor lithography is relaxed. Thus, the line widths and substrate thickness may be varied to provide a variety of designs and thereby accommodate a wide range of impedance requirements that would not be possible using the conventional interconnect structures discussed above.